The present invention relates to a noise measuring system for measuring noise of a mobile noise generator, like a vehicle or a mobile body, by using a plurality of microphones, and, more particularly, to a noise measuring system and method which can isolate a plurality of mobile noise generators.
In the following description of a noise measuring system to which this invention is directed, terms are defined as follows. "Direction of directivity" is the direction in which one wants to collect sounds. "Axis of microphone array" or "microphone array axis" is the layout line of a plurality of microphone units arranged on the same line. "Apparent wavelength" is the wavelength of a spatial waveform when the spatial waveform can be detected along the microphone array from the spatial phase difference of the positions of the individual microphone units on the same line. "Layout angle" is either the angle that is defined by the surface of a road and the microphone array axis or the angle that is defined by the line of movement of a mobile noise generator and the microphone array axis.
A conventional noise measuring system will now be described with reference to FIGS. 1 through 4 using the above-defined terms. Directional arrayed microphones comprising a plurality of microphone units laid on the same line has been used to measure and isolate the noises of individual mobile bodies which move on a plurality of lines of movement.
To improve the directivity of directional arrayed microphones, the number of microphone units is increased by arranging two microphone arrays in a V shape or arranging microphone units in an arc as proposed in Jpn. Pat. Appln. KOKAI Publication No. 4-324324. FIG. 1 shows the structure of the conventional directional arrayed microphones disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-324324. Referring to FIG. 1, sounds of a mobile noise generator 54 are collected by a microphone array 52 comprising a plurality of microphone units 51. The outputs of the individual microphone units 51 of the microphone array 52 are sent to a signal processor 53.
The principle of the directional arrayed microphones will be discussed below. The directional arrayed microphones comprise a microphone array comprised of a plurality of microphone units arranged on the same line and a computer for processing data.
With f representing the frequency of sounds which are measured by the individual microphone units arranged in a line, if the frequency f is known, the wavelength .lambda..sub.1 of a sound wave can be find out from the frequency f.
With the speed of sound being c, an equation (1) represents the relationship between the wavelength .lambda..sub.1 of a sound wave and the frequency f. EQU .lambda..sub.1 =c/f (1)
Further, the spatial waveform can be detected along the microphone array from the spatial phase difference of the positions of the individual microphone units on the same line. The wavelength of that waveform is defined as an apparent wavelength .lambda..sub.2. The angle of the approaching sound wave is acquired from the ratio of the wavelength .lambda..sub.1 of the sound wave to the apparent wavelength .lambda..sub.2 obtained on the microphone array.
An equation (2) shows how to acquire the angle of the approaching sound wave from the ratio of the wavelength .lambda..sub.1 of the sound wave to the apparent wavelength .lambda..sub.2 obtained on the microphone array. EQU .lambda..sub.1 /.lambda..sub.2 =cos .theta. (2)
FIG. 2 geometrically illustrates the principle of directional arrayed microphones. In FIG. 2, a numeral "58" indicates the direction of propagation of a sound wave, a numeral "59" indicates the wavelength .lambda..sub.1 of the sound wave, a numeral "60" indicates the apparent wavelength .lambda..sub.2 and a numeral "61" indicates the axis of the microphone array.
Collecting sounds at a specific ratio of the wavelength .lambda..sub.1 of a sound wave to the apparent wavelength .lambda..sub.2 according to this principle can allow sounds from a restricted direction to be output loud.
The direction of directivity (characteristic) of directional arrayed microphones is distributed symmetrical to the axis that is the microphone array, and forms a plane. This plane is defined as a directional plane.
Arrayed microphones whose directivity is formed by a delay time calculator will have the strongest directivity if the directivity is designed in such a way that the directional plane becomes a flat plane in a direction perpendicular to the microphone array.
FIG. 3 shows the directional plane of directional arrayed microphones available on the market, and a numeral "62" is the directional plane. FIG. 4 exemplarily shows measurement of noise of a mobile body by means of directional arrayed microphones available on the market. In FIG. 4, a numeral "64" denotes a line of movement.
The prior art has the following shortcomings. The directional plane of available directional arrayed microphones is a flat-like plane with the microphone array as a normal line. If a beam is put hanging above a road and the arrayed microphones are placed in such a manner that the microphone array is parallel to the road's surface and the projectional line of the microphone array on the road is perpendicular to vehicle lanes, therefore, a plurality of mobile noise generators which move on a target lane for measurement cannot be isolated.
If the arrayed microphones are placed in such a manner that the microphone array is parallel to the road's surface and the projectional line of the microphone array on the road is parallel to vehicle lanes, a plurality of mobile noise generators which move in parallel on lanes adjacent to a target lane for measurement cannot be isolated.
Further, the scheme of improving the precision of isolating mobile noise generators as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-324324 increases the number of microphone units that constitute the microphone array.
Increasing the number of microphone units however complicates the overall system and thus undesirably results in an increased computation time and an increased cost.
Accordingly, it is an object of the present invention to provide a noise measuring system and method capable of improving the precision of isolating mobile noise generators without increasing the number of microphone units.